After a week of 70%+ humidity, I’m seeing a persistent -60 dB hum on the program bus of our GL2400, independent of fader position and with 150 Ω terminations on inputs. Power is on a dedicated 20A circuit with star ground; cleaning the chassis bonds and I/O shells with 99% IPA only buys about 3 dB, and lifting audio grounds adds broadband hash. Before I open the PSU and start swapping caps, has anyone fixed a similar seasonal hum with transformer isolation or by re-bonding racks to the technical ground using serrated washers?
Try bypassing the L/R insert normals with a TRS jumper (tip to ring) or just exercise those jacks — humidity makes those contacts act like tiny antennas. “Tip-to-ring jumper on L/R insert” is my go-to quick test on GL desks; I’ve had a -55 dB hum vanish after a few plug cycles and a shot of DeoxIT. If no change and the hum’s at mains frequency, I’d scope the ±17 V rails for ripple before cracking the PSU.
Before cracking the PSU, pull the master section and reseat the long mix-bus ribbon — August humidity oxidized mine and I had a stubborn −60 dB on a GL2400 until a quick DeoxIT/press cycle fixed it. Also kill the lamp dimmer and pull the gooseneck once to rule out the lamp rail leaking into the bus when that pot films up. I’m with @amy_lu22 on contact weirdness, but the ribbon did more than jack exercising in my case.